Process for conditioning copper plating solution



PROCESS FOR CONDITIONING COPPER PLATING SOLUTION Henry George McLeod, St. Catharines, Ontario, Canada, assignor to I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application February 20, 1958 Serial No. 716,277

3 Claims. (Cl. 204-52) This invention relates to improvements in baths for the electrodeposition of copper on base metals and more particularly to improvements in aqueous alkaline copper cyanide baths containing additives adapted to plate smooth bright deposits of copper on base metal articles.

The present invention has particular application in the manufacture of chromium plated parts for the automotive industry where it is especially desirable to have smooth mirror-like finishes. The undercoat of copper must be very smooth without surface imperfections, such as scratch marks, to enable the final chrome plate to be free of such imperfections. Accordingly, it is necessary to initially have a smooth and bright copper coated base metal before subsequently plating the articles with additional coatings.

In practice the base metal usually has polishing scratches and other marks on it prior to electrodepositing copper. These surface scratches are often accentuated by the copper deposit which tends to preferentially deposit along the edges of the scratches. Thereafter, it is customary to bufi the copper-coated surface to remove these imperfections and considerable amounts of deposited copper are removed by bufiing before the article is suitably prepared for the final nickel and chrome deposits. Frequently the amount of bufiing results in bufling cut-throughs to the base metal. Not only is considerable copper wasted, but the cost of labor to buff-out these imperfections is considerable.

US. Patent No. 2,701,234 to Wernlund discloses a process of electroplating smooth copper deposits on base -metals using additives that induce leveling of the scratch marks on the base metal. Essentially, it has been found that the inclusion of certain amounts of water-soluble selenite or selenate salts and water-soluble salts of methylene-bis(naphthalene sulfonic acid) greatly assists to fill scratch marks without causing nodules, burning and other surface imperfections on the copper-plated article. The brightness of the copper deposits formed .using the Wernlund process is not as bright as those baths which employ selenides, especially copper selenides, in combination with the above-referred-to sulfonic acid compounds in conventional alkaline aqueous CuCN plating baths where the selenide is present within 2-50 p.p.m. and the bath contains at least 100 p.p.m. of the sulfonic acid compound.

The presence of more than 0.1 p.p.m. of selenide unfortunately destroys the leveling potential of the Wernlund process even when the additives of Wernlund are present. It has been recently found that the presence of minute amounts (0.1 to about 0.01 p.p.m.) of selenide considerably augments the brightness of copper deposits using the Wernlund process without destroying the leveling potential of the combination of additives disclosed by Wernlund.

The concentration of selenide ions must be maintained with the range of 0.01-0.1 p.p.m. to impart brightness to the copper plating without destroying the leveling Patented Nov. 17, 1959 potential of the combination of selenates or selenites together with the sulfonic acid compound.

It has been found that the addition of potassium bromate is an effective additive in controlling the selenide concentration and, when added in amounts of 0.1-5.0 g./l. to copper plating baths containing the leveling additives of Wernlund together with selenides in effective amounts, insures that both the leveling and brightness potentials of these additives will not be substantially diminished.

Baths containing up to 50 p.p.m. of selenide ions are suitable for increasing the brightness of copper deposits. The addition of potassium bromate within the range of 0.1-5.0 g./l. effectively lowers the selenide ion concentration to less than 0.1 p.p.m., permitting the use of Wernlunds process of leveling copper deposits.

It is an object of the present invention to provide an aqueous alkaline copper cyanide plating bath containing selenium additives capable of plating bright copper deposits free from scratch marks, which deposits require a minimum of bufling.

It is another object of the present invention to provide a means of maintaining the selenide ion concentration below 0.1 p.p.m. in copper plating baths adapted to plate level and bright copper deposits.

It is a further object of the present invention to lower the selenide ion concentration to 0.1 p.p.m. or less using potassium bromate.

It is a still further object of the present invention to provide a means of converting copper plating baths containing selenide ions to plating baths adapted to plate level and bright copper deposits by maintaining the selenide ion concentration between 0.1-0.01 p.p.m.

The above and other objects are accomplished by employing controlled amounts of selenium and other additives that induce leveling and adding 0.1-5.0 g./l. potassium bromate to curtail the anti-leveling properties of selenide ions when present in concentrations of more than 0.1 p.p.m.

The selenide additive is preferably in the form of copper selenide and between 2-50 p.p.m. may be used. The same amounts of selenites or selenates as disclosed in U.S. Pat. 2,701,234 are suitable.

'In addition to the selenium additives, a secondary brightener should be included in the bath. The secondary brightener is the bath-stable organic compound methylene-bis(naphthalene sulfonic acid) referred to in US. Pat. 2,701,234, referred to herein as MBS. The term MBS embraces the alkali metal salts thereof as equivalents of the acid.

The quantity of MBS is not sharply critical, particularly 1n the upper limit. Between about 200 p.p.m. and 1,000 p.p.m. are preferred, although p.p.m. up to 2,000 p.p.m. may be used.

As a conventional copper plating bath without additives, the following is typical:

Potassium hydroxide 1-6 oz./gal.

Free potassium cyanide 0.5-4 oz./gal. but preferably 1-2.

Copper cyanide 3-20 oz./gal. but

preferably 6-12.

The free cyanide is that measured by the Liebig silver nitrate titration.

within the above ranges are present, namely, from about 3-100 a./s.f.

Frequently, the plating current should be reversed periodically where the base being electroplated is made cathodic for about 60 seconds and then made anodic for about -20 seconds. The current can also be periodically interrupted for a second or more. These measures tend to allow use of higher current densities and produce better deposit distribution.

For example, a 30-gallon plating solution containing about 12 oz./ gal. of copper cyanide, 3 oz./ gal. of sodium hydroxide, 3 oz./gal. of sodium carbonate with a sodiumzpotassiurn ratio of 1:1 and a free cyanide of 1.5 oz./ gal. is used to plate copper on ferrous base metals. To this bath there is added sufficient cuprous selenide to give a concentration of about 10 p.p.m. of selenide ion and about 200 p.p.m. of MBS. Extremely bright copper deposits are obtained. However, scratch marks remain. In an effort to overcome the anti-leveling effects of this bath, 50 p.p.m. of sodium selenate is added. The results clearly show that contrary to expectation there is no leveling obtained on addition of sodium selenate in solution with MBS.

However, when 2 g./1. of potassium bromate is added for each gal. in the plating bath, leveling is restored. The concentration of selenide ion after treating with bromate is less than 0.1 p.p.m. A higher concentration of selenide ions in the same bath requires the addition of more potassium bromate. Although 3 oz./gal. of sodium carbonate is present, a concentration up to 9 oz./gal. apparently displays no anti-leveling characteristics. Amounts of sodium carbonate up to 9 oz./gal. are not critical and have had no adverse effect in the process disclosed herein.

Example 1 A commercial cyanide-copper plating bath containing about 10 oz./gal. copper cyanide, 3 oz./gal. potassium hydroxide, several (about 3 oz./gal.) oz./gal. sodium carbonate and a free sodium cyanide of 1.0 oz./ gal. had been operated as a bright copper plating bath for about 6 months using cuprous selenide and methylene-bis- (naphthalene sulfonic acid) as brighteners. Work plated in this bath using current reversal technique showed negligible leveling. Addition of the sodium selenate leveling agent failed to produce leveling. Extensive electrolysis of the solution (exceeding 200 a. hrs/gal.) followed by addition of the leveling agent still failed to produce leveling. Carbon treatment, carbonate removal, addition of hydrogen peroxide and other com monly used treatments all failed to give a solution which would produce leveling on addition of the sodium sele nate leveling agent. A fresh sample of the commercial bath was used to copper plate a test piece and negligible leveling was obtained. One g./l. potassium bromate was added to the bath and a second test piece plated. Roughness was reduced from an initial R.M.S. of

microinches to a final value of about 10 microinches in a 0.001" thick deposit. The copper deposit showed good leveling. An additional 1 g./l. potassium bromate was added to the bath and a further small improvement in leveling was noted.

The addition of potassium bromate was subsequently carried out in the large commercial bath from which the above solution was obtained and excellent leveling was obtained. Plating automotive bright trim using the bath which had been converted to a leveling plating bath by the addition of about 1 g./l. potassium bromate resulted in leveled deposits. Subsequent buffing to a final smooth finish removed less than half (0.00015 inch) the copper that was removed from deposits obtained before bath conversion (0.00035 inch).

Example 2 A cyanide copper plating bath containing about 10 oz./gal. CuCN, 3 oz./gal. KOH, about 3 oz./gal. of sodium carbonate and a free sodium cyanide of 1.0 oz./gal. was prepared. Fifty p.p.m. selenium as sodium selenate and l g./l. methylene-bis(naphthalene sulfonic acid) were added to the bath. A test specimen plated in this bath using a plating time of 30 minutes using a current reversal cycle of 60 seconds plating 20 seconds deplating showed good leveling. The deposit was somewhat hazy. Ten p.p.m. cuprous selenide was added to the bath. Another test piece was plated as above and negligible leveling was obtained. About 1 g./1. potassium bromate was added to the bath and the solution stirred for a few minutes. Another test piece was plated as above. The deposit showed good leveling and was now bright.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as fol lows:

1. In an aqueous alkaline copper cyanide plating bath containing selenide ions in excess of 0.1 p.p.m., the improvement comprising inclusion in said bath of 0.1-5.0 g./l. of potassium bromate.

2. In the process of electrodepositing copper from an aqueous alkaline copper cyanide plating bath containing selenium additives, the step of adding 0.1-5.0 g./l. potassium bromate to said bath and maintaining the selenide ion concentration at less than 0.1 p.p.m.

3. In the process of electrodepositing copper from an aqueous alkaline copper cyanide plating bath containing selenium additives, the step of adding potassium bromate to said bath in an amount suificient to maintain the selenide ion concentration within the range of 0.1-0.01 p.p.m.

Wernlund Feb. 1, 1955 Nobel et a1. Feb. 26, 1957 

3. IN THE PROCESS OF ELECTRODEPOSITING COPPER FROM AN AQUEOUS ALKALINE COPPER CYANIDE PLATING BATH CONTAINING SELENIUM ADDITIVES, THE STEP OF ADDING POTASSIUM BROMATE TO SAID BATH IN AN AMOUNT SUFFICIENT TO MAINTAIN THE SELENIDE ION CONCENTRATION WITHIN THE RANGE OF 0.1-0.01 P.P.M. 